Sal's Dune Buggy Rentals rents dune buggies at a minimum rate of 42 vehicles per month on January 1 and a maximum rate of 138 vehicles per month on July 1. Assume this pattern repeats every year. a) Find a sinusoidal model for f(t), the rate of dune buggy rentals t months after January 1. b) Find f'(t). c) How fast is the rental rate changing on April 1? d) How fast is the rental rate changing on June 15? a) What is a sinusoidal model for f(t), the rate of dune buggy rentals t months after January 1? f(t)=(Type an exact answer in terms of x.)